Labor unions and Democrats slammed the White House Thursday for weighing in against public-sector unions in a major Supreme Court case, saying the Trump administration's opposition to mandatory union dues threatens the unions' ability to represent government workers.

"While President Trump boasts his support for working families, his administration is advocating a position in the United States Supreme Court that disregards decades of settled law and threatens our livelihoods.

In a friend of the court brief for the case Janus v. American Federation of State, County and Municipal Employees filed late Wednesday, the White House argued that the mandatory dues violated union members' First Amendment rights and should be abolished.

Neither statement delved into the facts of the case, which involve if Mark Janus, an Illinois state government employee, can be forced to regularly pay a "security fee" to his workplace's union as a condition of employment.

In the brief, the administration argued that because the unions negotiate with governments over issues involving budgets and size of government, they by definition involve political viewpoints and therefore workers can be forced into subsidizing views they disagree with.